The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, a conventional engine is configured such that only one injector is provided for each intake port. Particularly, an MPI (Multi Point Injection) engine is an engine, in which a fuel injection valve is installed on each cylinder, and which is operated in such a way that fuel is injected in advance in each intake manifold. The MPI engine employs a system in which two intake valves and two exhaust valves are provided for each cylinder and a single injector is used for the two intake valves.
Unlike this, an engine with a dual port injector is configured such that two injectors inject fuel into the same cylinder. The two intake ports may communicate with the interior of the same cylinder, and two injectors may be disposed behind intake valves provided to open or close the respective intake ports. In the case where the dual port injector is used, there are advantages in that the fuel efficiency can be enhanced by an increase in volume efficiency, and the harmful material content in exhaust gas can be reduced.
Meanwhile, an injector has a minimum injection time, and the fuel injection time of the injector cannot be controlled to a value less than the minimum injection time. In the case of a low-load driving condition, particularly, in the case of a high-temperature idle or high-concentration purge state, a required injection fuel amount is small, so that the fuel injection time must be controlled to be short. However, we have discovered that if the minimum injection time of the dual injector is not sufficiently short, the required injection time may be shorter than the minimum injection time. In this case, there is a problem in that the fuel injection amount cannot be controlled in a very small amount of fuel.
In the conventional technique, an air-fuel ratio control operation is inactivated, and the fuel injection time is fixed at the minimum controllable injection time of the dual injector. As a result, the amount of injected fuel becomes larger than the required fuel injection amount. For example, in the case where the minimum injection time of the injector is 1.0 ms, and the fuel injection amount distribution coefficients of two injectors in each cylinder are the same as each other and a required injection time of each injector is 0.7 ms, in the conventional technique, each injector injects fuel for a fuel injection time of 1.0 ms greater than the required injection time. Consequently, it is difficult to precisely control the air-fuel ratio.